The present invention relates to a composite structural member, especially constructed for taking up dynamic loads. The load applying element is connected to the load take-up element through force transmitting means of elastic material glued to said elements along the surface thereof. The force transmitting means may comprise one elastic material element or a plurality of such elements arranged in groups. Composite structural members of this type are especially used in connection with testing the sheet metal forming the skin of the body of an aircraft or the like, whereby such sheet metal skin constitutes the force take-up element and the force applying element is connected to a source of power generating the dynamic testing force, whereby the just described structure forms a fatigue or limit testing cell for such aircrafts or the like.
It is known to construct composite structural elements by employing various gluing techniques. Thus, it is known to glue without any pressure the load applying hardware and the load take-up plate to a rubber block by means of a reaction glue free of any solvent. For example, modified epoxy resins or so called polyurethane precursors in the form of a two component adhesive are suitable for this purpose. However, the fatigue or breaking strength of these glued connections is frequently inadequate, for example, where such component structural members are used in connection with fatigue testing cells in aircrafts or the like. For this purpose, it is necessary to connect the rubber block to the sheet metal forming the skin of the aircraft without using stiffening means for the skin sheet metal, which would falsify the measured results. Moreover, the glued connection is frequently exposed to very large dynamic loads within unfavorable frequency ranges, especially where longivity tests are performed. This is so, even if several load transmitting elements, such as said rubber blocks are arranged in groups in order to distribute the load application over a larger surface. The number of rubber blocks would, of course, be selected with due regard to the load to be transmitted.
It is also known to secure the load transmitting element to the two other elements by means of a reaction molding method, which employs a cold curing, viscous flowing mass of a polyurethane prepolymer. This mass is directly cast onto the skin sheet metal, whereby the mass enters into an intimate bond with the sheet metal, as well as with the load applying hardware or element. The adhesion between such polyurethane elastomers and the light metal surface reaches rather high values. However, the load strength of such composite structures is also rather limited, because breaks or fractures in the cohesion have been encountered where large dynamic loads or increasing tension loads are applied to these structures. Furthermore, such castable masses have a limited shelf life. Besides, these masses tend to form shrinkage cavities due to side reactions so that these known composite structures are mostly used where relatively small tension loads are expected to be encountered.
It is also known that molded blanks of polyurethane having an open cell porous structure which are produced by pressure casting or molding and simultaneous heat application, have a high tensile strength and a large breaking elongation. In addition, these molded blanks have a substantial resistance against tearing, especially against the transmitting of a tearing throughout the blank. Their resistance against oxygen, ozone, oil and fuel such as gasoline is also well known. In addition, these molded blanks have a fatigue or breaking strength as well as a normal strength and permanent deformation characteristics similar to those of rubber type materials. Besides, the relatively high hardness combined with a simultaneous large elasticity is unmatched by any other elastomeric materials. In view of these advantageous material characteristics of open cell polyurethane molded blanks, this type of material is well suited for avoiding a cohesion failure, even in connection with so called longivity load tests, such as are performed, for example, in connection with aircraft failure test cells which are exposed to a number of load changes exceeding 10.sup.7, whereby these materials still avoid failures. However, so called adhesion failures between the load input element and the load transmitting element, as well as between the latter and the load take-up element are not uncommon due to the limited adhesion strength and the limited resistance against peeling of the glued connection.